When glass batch compositions are melted in a glass melting furnace, volatile components can be released from one or more of the glass batch materials. As used herein the terms “glass batch” or “glass batch composition” mean one or more glass batch materials that when melted form a specified glass composition. In particular, when glass batch materials are melted to produce certain types of glass composition, e.g. “E-glass”, volatile components, such as boron, fluorine and/or sulfur-containing compounds, are released into the furnace atmosphere. Depending on the temperature and humidity of the atmosphere, these volatile components can form gaseous compounds such as HF, SO2, and H3BO3 or be condensed to form solid compounds such as HBO2. The loss of such volatile components from the glass batch not only increases batch cost but also creates problems when the exhaust stream is vented to the atmosphere. For example, at high temperatures, these volatile components can form highly corrosive acid gases that become entrained in the exhaust system. As the exhaust stream cools, condensation of corrosive acids and other undesirable particulate materials can occur in the exhaust system causing deterioration of system components, increased maintenance costs, reduced operating efficiencies and emissions control problems. For example, the condensation of gaseous H3BO3 to form a sticky, solid particulate (HBO2) has been found to clog ductwork and filtering systems and contribute to visible emissions (or opacity) of the vented exhaust stream. While all of these volatile components present certain emissions control issues, of particular concern are the volatilized boron compounds that are difficult to control and recover.
Typically, attempts to reduce or eliminate volatile components from an exhaust stream involve the use of specialized wet or dry scrubbing processes or a combination of both. U.S. Pat. No. 4,208,201 discloses a process wherein dust from a batch house is introduced into an exhaust stream from one or more melting furnaces. The dust particles, which preferably have a diameter of ten microns or more, form nuclei upon which condensables in the exhaust will condense (col. 2, lines 38-44). After mixing with the exhaust, the dust particles are filtered from the exhaust stream and returned to the batch source and reused (col. 2, lines 67-68 and col. 3, lines 1-2). U.S. Pat. Nos. 3,995,005 and 3,969,482 disclose methods of treating flue gas from a melting furnace using a two-stage process comprising a first step of quenching the flue gas with an alkaline solution or slurry of basic material to form a salt and a second step of contacting the flue gases with a particulate sorbant material to remove residual acid gas. Preferably, the temperature of the flue gas ranges from about 200° F. to about 300° F. (about 93° C. to about 149° C.) immediately prior to mixing with the sorbant material. Additionally, it is preferred that the concentration of residual acid gas in the flue gas is reduced to less than about 500 parts per million prior to mixing with the sorbant material since the sorption process is generally not economical to employ at higher concentrations (col. 7, lines 33-38 of U.S. Pat. No. 3,969,482). It is also preferred that the temperature of the gas stream introduced into the bag house be below about 185° F. (about 85° C.) to minimize the volatility of the boric acid deposited in the bag filter.
Such two step processes are complex, expensive and can be difficult to operate and maintain. Additionally, it has been observed that the recovery of boron compounds by condensation, such as by the introduction of flue gas containing volatile boric acid species into bag filters at temperatures less than about 190° F. (about 88° C.), can lead to clogging of ductwork and bag-blinding due to the deposition of sticky boric acid condensates thereon. As used herein the term “bag-blinding” means that the filter bag becomes coated or clogged such that airflow through the bag is severely restricted. Furthermore, little or no recovery of energy from the flue gas is achieved in such a system.
Other patents have been directed toward the recovery of energy, particulate materials and volatiles from a flue or exhaust gas stream of a melting furnace by passing the exhaust stream through a bed or column of pelletized batch materials. U.S. Pat. No. 3,953,190 discloses a preheater and recycling structure having a glass batch pellet containing intermediate section through which hot exhaust gas is passed. As the exhaust gas passes through the structure, the pellets are heated and the gas stream is cooled to permit the condensation of volatile materials and dust therein (col. 3, lines 31-35). The temperature of the gas entering the structure ranges from about 1000° F. to about 1600° F. (about 538° C. to about 871° C.) and is cooled to about 600° F. (about 316°) upon passing through the structure and is vented at a temperature of about 450° F. (col. 4, lines 6-13). The preheated pellets are subsequently fed into the melting furnace. U.S. Pat. No. 4,248,615 discloses a process for recovering energy and abating pollution in a glass manufacturing process, wherein flue gas from a melting furnace is directed into a preheater containing agglomerated batch materials to heat the agglomerates prior to their introduction into the furnace. After passing through the preheater, the gas is passed into one or more preconditioning chambers to preheat agglomerated batch materials prior to their introduction into the preheater. Particulates can be separated out of the flue gas due to the “filter-type” action of the agglomerates (col. 6, lines 7-8). Additionally, some gaseous polluting species can be recovered due to condensation as the temperature of the flue gas is decreased (col. 6, lines 11-15).
While such methods and apparatus are convenient for use with pelletized batch materials, they tend to be inefficient in recovering volatiles due to the low active surface area associated with agglomerated or pelletized materials, and are not well suited for use with particulate batch materials due to difficulties associated with passing an exhaust stream through a bed of particulate material. For example, passing a hot exhaust stream through a bed of non-agglomerated, particulate materials can result in the generation of dust and the loss of fine particles, as well as the formation of aggregates and high system pressure drops. Particulate glass batch materials also tend to be difficult to fluidize due to their fine particle size.
U.S. Pat. Nos. 4,298,369 and 4,282,019 disclose systems for preheating pelletized batch materials with flue gases while improving the removal of volatile species from the flue gas. U.S. Pat. No. 4,298,369 discloses a glass manufacturing process, wherein a particulate boron and/or fluorine reactive material is introduced into and reacted with a flue gas stream at a temperature in excess of about 500° C. (about 932° F.) (col. 2, lines 1-8). Preferably, the reactive material is added to the flue gas, on an oxide basis, at such a rate that a weight ratio of the oxide to the total boron and/or fluorine flowing in the gases coming from the recuperator will be at least 4 and more typically 5-10 times that ratio (col. 5, lines 17-24). The flue gas is then passed through a slag box to remove large particles and then through a bed of pelletized batch material to preheat the pelletized batch material, preferably to a temperature of about 500° C. (about 932° F.). U.S. Pat. No. 4,282,019 discloses a process of calcining colemanite, abating pollution and preheating pelletized batch materials, wherein raw colemanite is introduced into a flue gas stream at a temperature in excess of about 500° C. (about 932° F.) to decrepitate and react the colemanite with volatile boron and/or fluorine in the gas. The gas and colemanite are then passed through a cyclone separator to separate and recover the colemanite. After separation the gas is passed through a pellet preheater. Preferably, the temperature of the gas passing through the pellet preheater will be in excess of 500° C. (about 932° F.) (col. 3, lines 58-63).
Again, processes are not well suited for use in systems wherein non-pelletized batch materials are fed into a melting furnace due to difficulties associated with passing an exhaust stream through a bed of particulate materials (as discussed above).
Attempts have been made to preheat particulate materials using exhaust gas. U.S. Pat. No. 4,099,953 discloses the use of a fluidized bed preheater to preheat starting material for a glass batch composition. Exhaust gas is passed from a melting furnace into a fluidized bed to preheat the starting materials contained therein. A high performance filter is used to collect fine particles entrained in the residual gases of the fluidized bed preheater. U.S. Pat. No. 4,349,367 discloses a method of recovering waste heat using a granular heat exchange medium, wherein exhaust gas is passed through a first bed of granular material to recover heat therefrom. The heated granular medium is then passed into a second bed where it is used to preheat combustion air. Particulates in the exhaust stream can be recovered by the granular heat exchange medium of the first bed or they can be filtered prior to passage through the first bed by contact with a bed of cullet material. The cullet material can then be passed into the melting furnace. However, neither of these patents address the recovery of volatile contaminates from the exhaust stream.
Accordingly, there is a need for an effective method of reducing and reclaiming a variety of volatile components, particularly volatile boron compounds, from an exhaust stream that can be used in conjunction with a particulate batch feeding system and that provides for reduced system complexity, reduced batch costs, increased utilization of energy and improved bag house operations.